This invention relates to subtraction fluoroscopy and radiography.
Subtraction of x-ray images is a known method for making low contrast structures in the images more conspicuous. In ordinary x-ray images, the bone, blood vessels or other tissue of primary diagnostic interest may be surrounded or overlayed or underlayed by tissue or bone which obscures and prevents easy visualization of the structure of interest. The function of the subtraction process is to remove or suppress the potentially confusing effects of uninteresting overlying and underlying structures to thereby enhance the detectability of the structure of interest.
Image subtraction has been used in the past primarily for angiography, that is, for making x-ray examinations of blood vessels such as the vessels of the heart. In accordance with this technic, a radiographic or fluoroscopic x-ray image of the heart is made and recorded. The first image is called a mask. Shortly afterwards, an x-ray opaque medium, such as an iodine compond, that has been injected into the blood vessels reaches the vessels of interest and then another image is made. The blood vessels of interest in the pre-injection and post-injection images are almost always obscured by overlaying or underlying bone or other tissue, thus making visualization difficult. However, when the images are subtracted from each other, anatomical structures which cause confusion are deemphasized or substantially eliminated and a high contrast and more easily visualized image of the iodine-infused vessels remains.
Obtaining a mask image and one or more images in sucession at a relatively low rate is characterized as temporal subtraction. It is satisfactory for dealing with anatomy whose position is static or slowly changing but there is often a significant loss of registration between successive images when the vasculature of a rapidly moving organ such as the heart is being examined. This is manifested by blurring and loss of detail in the subtracted image. Even motion of anatomical regions due to such things as peristalsis and breathing can produce motion artifacts in temporally obtained subtraction images. For many applications, differentiation of low contrast anatomical structures can be obtained by acquiring images at two or more different x-ray energy levels. For instance, it is known that the mass attenuation coefficient of bone and soft tissue is much lower at an x-ray photon energy level corresponding with about 70 peak kilovolts (kVp) being applied to the x-ray tube than is the mass attenuation coefficient of iodine at the same energy level. It is also known that as one progresses up the energy scale such as to 135 or 140 kVp, the mass attenuation coefficient of soft tissue changes by a relatively small amount, but the iodine changes by a large amount. Thus, it has been proposed, as in co-pending application of L.S. Edelheit, Ser. No. 179,203, filed on Aug. 18, 1980, owned by the assignee of this application, to use an x-ray image intensifier for producing a quick succession of low and high kVp images. The successive images are viewed on the output phosphor of the image intensifier tube by a single video camera and the analog waveforms for each of the images are digitized and stored in separate memories. The picture elements (pixels) stored in corresponding locations in digitized form in the two memories are then combined to produce data for an image with enhanced contrast but with certain intensity levels, such as those due to bone and soft tissue of little interest, being suppressed. In this system, the single image pick-up tube or video camera which is used is blanked during x-ray irradiation and scanned or read out after each irradiation. However, the long time response of any video pick-up device relative to the time interval between exposures has a tendency to create overlapping images and would, by itself, lead to relatively poor quality subtracted or combined images.
A method of mitigating the problem involves scrubbing the video camera target plate during retrace time with a very high electron beam current. The beam deflection power necessary to scrub all previous raster lines in the one millisecond or so that is allowed, forbids such an approach with presently available standard video cameras. The use of time for scrubbing and reading imposes two limitations on the single image pick-up device system. First, the maximum image acquisition rate is about 10 frames per second. Second, the high and low energy x-ray pulses must be separated by at least two frame times or about 70 ms. Due to the substantial lapse of time between the high and low energy x-ray pulses, there is a greater likelihood that the anatomical structures will have moved so that undesirable loss of registration between pairs of subtracted or combined images mentioned earlier will occur. Furthermore, on some occasions, such as when the physician desires to watch progression of the opaque medium continuously and in real-time over an interval of twenty seconds or more or where the frame rate must be high enough to produce the effect of stopping heart motion, no time is available for scrubbing when a single pick-up device such as a video camera is used.